In the photomechanical arts, a mask containing an image that is opaque to actinic radiation is used as an intermediate or "master" in preparing a printing plate of some kind; whether letterpress or lithographic, the method of producing the plate is much the same. A metal (or in some cases, plastic) plate coated with a photosensitive resist-forming material is exposed to actinic light through the mask. After exposure, the plate is developed by treatment with a solvent that removes either the exposed or the unexposed material. This leaves a resist image which protects the resist-covered areas of the printing plate from the acid used in etching the plate (in the case of letterpress) or from the various hydrophilic coatings used in lithography. Relief and lithographic printing plates may also be prepared directly from such masks, as exemplified in U.S. Pat. No. 2,760,863 and U.S. Pat. No. 3,458,311.
For production of such masks, "litho" (for lithographic) films are preferable, because they employ photosensitive silver halide emulsions which are capable of producing extremely sharp images, i.e., images of high density and contrast. Litho films of this type permit the continuous density variations in the original transparency or image to be rendered as a halftone image consisting of an array of opaque dots of variable sizes or areas and complementary transparent areas. Such processes of forming litho masks are described in Chapter IV of "The Printing Industry" by Victor Strauss, published by Printing Industries of America (1967). The tonal characteristics of the image (tone referring to gradations within a color hue) are determined by the dot size, i.e., the percent of the surface area in any small region covered by halftone dots. Reducing or enlarging this percent by reducing or enlarging the area of the halftone dots thereby changes the tone values of the image. This principle is widely applied in lithography, and is important in correcting color work. It thus permits tone values or color strength to be changed during the photographic steps rather than on the printing plate.
When producing a mask from a litho film, the latter is exposed through a vignette halftone screen and a continuous tone image transparency and then developed, thereby forming a halftone image consisting of an array of opaque dots on completely transparent areas.
It is known to effect dot size reduction, or dot etching, with conventional silver halide litho film by treating the film area with an oxidant for the silver in the dot image, but there is no similar direct method for enlarging such dots. One method of dot enlargement entails touch-up of a continuous-tone transparency by manually adding density, e.g., with an air brush or pen and ink, and then producing a completely new halftone mask therefrom. A second method of dot enlargement is to produce a halftone negative of the original halftone mask, dot etch the negative, and from the dot-etched negative produce a second halftone mask which is then a reproduction of the original mask but with enlarged dots. Both of these methods are expensive and time consuming.
It is also known to effect dot size reduction, or "dot etching", in photopolymer lithographic masks; this is described in U.S. Pat. No. 4,173,673. In addition to the methods used for silver halide masks, dot enlargement of photopolymer masks has also been achieved by two other methods: (1) by a burnishing technique, (2) by treating the film area with a solution of a swelling agent and a fatty acid, followed by stabilizing the swollen dot image by treatment with an aqueous solution so as to precipitate the salt of the acid within the matrix. However, the swollen image shrinks partially over a period of time, and the burnishing method is useful only for small areas where uniformity is not critical. There is need for a better method of changing the tonal values of photopolymer masks by image enlargement, particularly for a method which will uniformly and irreversibly enlarge the polymeric dots of a halftone image mask by a simple treatment.